The homebrew developer "Extrems" claims to have found a bug in the Nintendo SDK used to compile retail Wii games that causes the graphics on 480p to look bad (not as good as they could look without that bug). More information on the bug can be found here, a patch for libogc (homebrew) can be found here.

I have ported said patch to apply to retail games (using Mario Kart Wii as an example): patch

I quickly tested it in Dolphin and didn't see any obvious improvement, but I can't test it on Wii so I don't know how huge the actual improvement would be. Maybe one of the more experienced Dolphin devs can take a look at this and check if this patch actually affects Dolphin as well / if this patch improves graphics quality on Dolphin as well, then it could possibly be added as a Dolphin setting to automatically apply that fix to a game? I don't exactly know how Dolphin does its graphics rendering / how much of the original Wii "features" it uses, and if this code would even be relevant for Dolphin ...